Problem. Mythbusters is (was?) a really fun, science-like TV show, and I highly

Question

Problem.

Mythbusters is (was?) a really fun, science-like TV show, and I highly recommend watching it. In this outtake clip, they also play with Rubens’ tube: http://www.youtube.com/watch?v=ynqzeIYA7Iw However, their science explanation are sometimes a bit superficial, and in this clip they make a pretty serious physics gaffe when they point out how to measure the wavelength based on the flames (and you now know enough to spot it!). Explain their error, and explain how you should measure the wavelength based on the appearance of the flames. (Hint: the more subtle part of their error is in their statement that they are looking at areas of low and high pressure reflected in the flames—technically, it’s the areas of changing and not changing pressure. Note that while I think this show is great, keep in mind that rigorous science and entertainment are not necessarily the same thing. And it’s possible to make pretty basic mistakes, even on a gazillion-dollar budget!)

http://www.youtube.com/watch?v=ynqzeIYA7Iw

Explanation / Answer

There are two ways of measuring wavelengths

1. points where the displacement is minimum(displacement nodes). These are points where pressure is greatest(pressure anti nodes).

2. points where displacement is maximum( displacement anti nodes). These are points where pressure is minimum (pressure nodes).

At displacement anti nodes, the flame is yellow and long and at displacement nodes, the flames are short blue and slightly agitated.

So your point about areas of constant/not changing pressure is correct. Either at pressure anti node or node the peak/trough is due to the pressure being maintained at that point. IF you take the derivative w.r.t to the distance, these points will equate to 0 as they are points of maxima or minima.

You can measure the wavelength using either of the two though. Whichever yields best results.

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.